The invention relates to a method and a plant for cleaning contaminated condensate emanating from pulp production of semichemical and chemical cellulose pulp, including evaporation of spent liquor.
When producing chemical cellulose pulps a lignocellulose material, for instance wood in the form of chips, is dissolved employing either an acid or alkaline process. During a cooking step lignin which forms the middle lamella between wood fibers goes into solution in a cooking liquid, and the fibers after digestion are separated from each other and form a cellulose pulp. Aside from the lignin content of the wood, a considerable part of the hemicellulose in the wood also goes into solution. How much is determined by the pulping degree, which in general is represented as cooking yield in percent. Also a minor part of the wood cellulose content goes into solution. An example of an acid cooking process is the sulphite process and an example of the alkaline cooking process is the sulphate process.
Other known alkaline cooking processes are the polysulfide pulping process and soda type (sodium hydroxide) process, in which catalysts such as quinone compounds can be used. Included within the sulphate process is utilization of high sulfidity pulping to counter current cooking, in which white liquor (primarily a mixture of sodium hydroxide and sodium sulphide) is added during the cooking phase and utilization of a chemical treatment of lignocellulose material, prior to the sulphate pulping process.
The sulphite method can be classified according to the base used in the cooking liquor, as calcium, magnesium, ammonium and sodium. There are sodium and magnesium cooking liquors, which are usually recovered.
After pulping the lignocellulose material, the cooking liquor is separated from the fibers. The spent cooking liquor known as black liquor is referred to as thin liquor in connection with recovery, the main part of thin liquor being water. The dry solid content in thin liquor (lignin, hemicellulose, cellulose, residual chemicals, etc.) is for instance within the range 15-20%. Before combustion of the thin liquor in a recover boiler with combustion of the organic compounds to mainly carbon dioxide and water with recovery of energy and permitting the inorganic compounds to form a residual in the form of a smelt along with recover of the liquor for production of new cooking liquor, the dry solid content must be increased to at least 55%. This liquor with the increased dry solid content is referred to as black liquor or thick liquor. The thick liquor is created by evaporation of thin liquor in five to seven steps. Each step is referred to as a stage.
Modern mills for production of chemical cellulose pulp attempt to reduce fresh water consumption as much as possible and also attempt to reduce the discharge of waste liquor. This may be achieved by closure of the liquid system to a higher or lower degree. This means that spent liquor from bleaching stages must be reused and may be mixed with spent cooking liquor. Thin liquor is therefore sometimes a mixture of spent cooking liquor and spent liquor from different types of bleaching.
During evaporation of thin liquor, condensate is generated. Condensate at some locations may be relatively clean, and can therefore be used at one of several positions in the pulp mill. At other locations contaminated condensate is generated. According to the present invention, it is imperative that heavily contaminated condensate be cleaned. Production of chemical cellulose pulp also generates other types of unclean condensate. Digesting of lignocellulose material is done under pressure such that after cooking a gas mixture is released from the digester and this mixture contains steam as well as organic- and inorganic compounds. During conventional batchwise cooking spent liquor leaves the digester together with generated cellulose pulp. The gas mixture may be condensed to a so-called cooking condensate. Gas mixtures are also collected at other places in the cooking plant and those are condensed, and are also known as cooking condensate and not blow steam condensate. Cooking condensate is generated during batch wise and continuous cooking of lignocellulose material. The contaminated cooking condensate is mixed with unclean evaporation condensate and that mixture may be cleaned according to the present invention. According to the present invention, it is possible to clean the condensate separately, but that is not preferred.
There are several semichemical pulping processes. An example is the neutral sulphite semichemical process, (NSSC). The pulping degree is very low for that type of process; mechanical defibration is therefore necessary for liberation of fibres. In some cases, the pre-treatment liquor or the cooking liquor is recovered. If the recovered liquor is evaporated, the present invention can be used.
For cleaning of unclean condensate according to conventional techniques, at least one stage called stripping is used. Stripping means that the unclean condensate is blown with steam such that volatile compounds in the contaminated condensate follow the steam flow and leave the condensate. According to conventional technique, separate or detached stripper plants are used. The steams used in the stripper plants are admission steams generated in the recovery boiler, or steam from any evaporator stage in the evaporation plant. Further, the stripper plants operate at atmospheric- or over pressure.
In Swedish letters patent 7704352-9 (423915) a method is described for recovery of sulphur compounds, volatile alcohols such as methanol and by-products such as turpentine or similar compounds from contaminated condensate. According to the described method, stripping is performed in two positions, one at the top of a rebuilt evaporation stage resulting in a more expensive evaporation plant and the other in a detached stripper. In the detached stripper, steam from the last evaporation stage is used for stripping of contaminated condensate. The use of the steam for the last evaporation stage provides an economic advantage. However the utilization of steam is limited to 20-25% of the total amount steam from the last evaporation stage. The surplus steam from the last evaporation stage is condensed in a conventional manner by using condensers and heat exchangers. As coolant, normally fresh water is used, resulting in production of warm water with low value.
The known technique for cleaning of unclean condensate, as described above, is uneconomical. The reason for that is that stripping is performed in separate or detached plants with the use of high value steam, mainly admission steam.
The present invention solves this problem and relates to a method for cleaning of unclean condensate arising from pulp production of chemical or semichemical pulp. The invention involves evaporation of spent liquor using a cleaning plant having in series several coupled condensers, characterized in that vapour generated at the last evaporation stage, i.e. process steam from the last evaporation stage which has the lowest steam pressure, and unclean condensate, feed a combined stripper and condenser, in which process steam and the unclean condensate flow counter current so that heat exchanging take place between the process steam and unclean condensate. This results in volatile compounds in the condensate being separated from the condensate and following the flow of process steam at the same time as indirect cooling occurs, resulting in condensation of the main part of the process steam. The rest or uncondensed portion of the process steam flows to another condenser, in which the remaining process steam successfully cools indirectly, resulting in water and turpentine condensing and collecting together. Methanol thereafter condenses. The first mentioned collected condensate is separated from the main part of the turpentine content, and is removed from the plant. Residual condensate is fed back to the combined stripper/condenser and the condensed methanol removed from the plant. Primary cleaned condensate collects at the bottom of the combined stripper/condenser and is also removed from the plant or will be further purified. It is highly stressed according to the invention, that all process steam from the evaporation plant last stage i.e. process steam with the lowest vapour pressure, feeds the combined stripper/condenser.
It is optimal if the amount of process steam from the last evaporation stage corresponds to the amount of steam necessary for purification of the unclean condensate, as described. A surplus of available process steam doesn""t create a problem, as the excess steam can be used for production of warm water according to conventional technique. A shortage of available process steam means that additional steam must be added i.e. admission steam, for achieving a good cleaning result. In some pulp mills more or less generated process steam can be used in other places in the mill, and therefore may not be available for purification of unclean condensate as described. If shortages of process steam arise, additional steam must be added i.e. admission steam. To proceed in that way is not preferred.
It is suitable for unclean condensate to enter into a space above and connected to the combined stripper/condenser and for process steam to enter a space under and in connection with the combined stripper/condenser, and for the unclean condensate to flow in the direction from the top and downwards through the combined stripper/condenser, while the process steam flows in the opposite direction. In the space above and in connection with the combined stripper/condenser, de-aeration steam can be introduced from one or more evaporation stages. In such cases there will not be any cleaning of condensate, but the reason for proceeding in that manner is to condense the unclean process steam and recover contaminating chemicals for purposes of environmental protection.
According to a preferred embodiment of the invention, the primary cleaned condensate flows downward through an underlying extension of a combined stripper/condenser and the primary cleaned condensate is heated indirectly with steam from de-aeration of one or more evaporation stages. The de-aeration steam enters the under part of the extension, resulting in any remaining volatile compounds separating from primary cleaned condensate as a gas and flowing upward and mixing with the upward flowing newly introduced process steam. Highly purified condensate is collected at the extension bottom and thereafter may be removed from the plant. In one embodiment of the invention, de-aeration steam can be reinforced with admission steam of variable energy content so that the cleanliness of the condensate can be controlled. It is important that the purified condensate be as clean as possible. The reason for that is that purified condensate is used in several places in the mill i.e. pulpwashing and in different positions in the bleachplant, and the condensate may also be sent into a sewer. Any remaining organic compounds in the purified condensate may have an adverse effect on the environment if the condensate is disposed of in a sewer. Condensate used in different positions in the mill may also have an indirect influence on the environment.
Conventional methods for cleaning of contaminated condensate have a cleaning efficiency of 90%. A higher cleaning efficiency is desirable and very significant.
By use of the mentioned admission steam, it is possible to further increase the already good cleaning efficiency which is achieved with the previous described embodiment. A cleaning efficiency of 95% and even higher is possible. It is true that adding of admission steam is against one of the important principals of the invention, namely that only low value steam, i.e. cheap steam, shall be used for cleaning of unclean condensate. However, the improvement that can be reached by means of the described addition of admission steam is very favourable in comparison with other cleaning alternatives and also is an advantage from an economical point of view.
The remaining part of the de-aeration steam, for indirect heating, leaves the extension part at the upper end. The surplus de-aeration steam enters the space at the top of the cleaning plant, the combined stripper/condenser. The reason for that has been described earlier.
According to another preferred embodiment of the invention, there is a vacuum creating apparatus connected to the discharge unit in the heating plant. This results in the removal of inert gases and foul smelling sulphur containing gases from the plant. By inert gases is meant that the gases will not condense under present conditions, not that the gases are chemically inert. These inert gases are sent to an incineration plant for destruction of the sulphur containing gases, according to known techniques. As low pressure is created in the device for cleaning of unclean condensate, process steam with temperatures below 100xc2x0 C. can be used. The temperature of the process steam can, for example, be 50-65xc2x0 C. Cleaning of contaminated unclean condensate is performed, as described earlier, in several steps and a suitable number of steps is four. The condensed methanol is collected in the bottom of the last condenser in series, a part of the condensed methanol enters a space above or in close connection with the next to the last condenser in series and the rest of the methanol is removed from the plant. The process steam removes impurities in the unclean condensate; the impurities are then recovered by condensation of the further contaminated condensate. To achieve condensation, coolant has to be introduced in to each step and heated coolant has to be removed in several positions. Accordingly the coolant is introduced at the upper end of the first condenser, i.e. the combined stripper/condenser and is removed at a higher temperature from the bottom end, or alternatively is removed at the middle section of the first condenser. Any coolant can be used, but cold water is preferable.
The invention also is applicable to a plant for cleaning of contaminated condensate emanating from production of semichemical or chemical pulp, including evaporation of spent liquor. In such an application the invention utilizes several in series of coupled condensers, including a first unit in the form of a combined stripper/condenser having several insertions, or insertion means, through which process steam and unclean, contaminated, condensate flow counter current and with direct contact between the process steam and the unclean condensate. The insertions are surrounded with a closed space for a cooling agent and a space above and under the unit. The space under the unit serves as a bottom. There are also means for supply of unclean condensate to the space at the upper end of the unit, means for supply of process steam to the space at the bottom end of the unit, means for supply of cooling agent at one end of the unit, means for removal of heated cooling agent at the opposite end of the unit, means for removal of primary purified condensate, further condensers for residual steam from the first condenser, having several insertions through which only process steam and steam generated from condensate flows, and not condensed process steam itself. The insertions are surrounded with a closed space for cooling agent. This results in condensation in series of contaminated water, and turpentine in a mixture and lastly methanol. There are also means for supply of a cooling agent at one side of the condenser and means for removal of the above mentioned condensate mixture from the space on the bottom end of the condenser and for separation of the main part of the turpentine content from the condensate mixture and for removal of turpentine from the plant, and for transporting remaining condensate back to the space in the first unit into which the unclean condensate flows. There is also means for removal from the plant of condensed methanol from the bottom side space of the subsequent condenser.
The insertions may be either tubes or lamella. The plant may also have means for supply of de-aeration steam from one or more evaporation stages to the space in the first unit where unclean condensate is introduced. According to a preferred embodiment of the invention a first unit in the form of a combined stripper/condenser has a downward extension having insertions through which primary cleaned condensate flows, which insertions are surrounded with a space for through flowing of a de-aeration steam from one or more evaporation stages counter current with the primary cleaned condensate. The extension has a bottom, from which high-grade cleaned condensate is removed according to previously mentioned means and further means for the supply of steam and removal of the residual indirect heating steam. The just mentioned insertions are preferably tubes.
According to one more preferred embodiment of the invention there is an arrangement for creating a negative pressure which is connected to the bottom of the last in series condenser. This permits removal of inert gases and foul smelling sulphur containing gases. The number of condensers connected in series is not critical, but it has been shown than an appropriate number is four, with the first as a combined stripper and condenser. By connected in series is meant that the condensers are connected in sequence with each other. For example, the first and the second condenser can be connected with a space at the top and above the condensers and the second and the third are connected with a space at the bottom end below the condensers and the third and fourth condensers are connected with a space above the condensers. The fourth or commonly the last condenser in series can either be placed in close connection with the next to the last condenser or it can be placed separate from the next to last condenser. In the first case a common cooling agent system can be used for all condensers, for example four condensers, while in the second case a special cooling agent system will be used, where methanol is collected and removed. This make it possible to use a powerful cooling agent system, which means that a number of sulphur containing gases will condense and will mix with the condensed methanol. In that way, the volume of the residual or inert gases which have to be collected at the end part of the plant, will be reduced, while at the same time the volume of condensed liquor will increase in that position. As the recovered methanol normally will be sent to some equipment for incineration, it is favourable to eliminate the sulphur containing agents, which otherwise will appear as very voluminous gases.
It is preferable to build all condensers, for example four, in a common surrounded space. However, the last condenser can be placed separately. According to the design, the invention comprises means for taking out part of the removed methanol for reflux to the space above the next to the last condenser in series.
To increase the stripping efficiency in the first unit, the combined stripper and condenser, it is suitable to apply means for increasing the surface contact inside the tubes i.e. the contact between the condensate and the stripper steam, process steam. The means for doing this can be two joint plates inside the tube creating any type of pattern, for example a cross. For further increasing the efficiency, the means can be twisted along the longitudinal axes creating a spiral form. The means can also be corrugated plates. The means can be in contact with the tubes inside the wall or can be placed a distance from the inside wall. It is of course possible to join more than two plates, for example three or four, creating different types of patterns.
According to the present invention it is preferred that the total amount of steam necessary for cleaning the unclean condensate be low value, cheap steam, which is available as a residual product from the evaporation of spent liquor. That contributes to low operating cost for the cleaning process. The fixed cost i.e. the investment cost for the cleaning plant will be reduced to a great extend depending on the design of the combined stripper and condenser. Further, according to the invention, it is possible to reduce the main part of the cleaning plant to one unit i.e. all condensers are enclosed for example in a cylindrical building, or a cylindrical tower. That implies on one hand that the cost of material will be reduced and on the other that the space necessary for the plant also will decrease. For pulp mills, which have a shortage of space, this fact is of big importance. Further, according to the invention, it will be possible to increase the cleaning efficiency to 95% and even higher which gives big environmental advantages. In some cases it can be necessary to use admission steam for reaching a high degree of cleaning, but for special cases it can be worth it in comparison with other measures.